Wind turbines convert kinetic energy of wind into electrical energy. In a wind farm, the first row of turbines that encounters the free-stream wind-speed can potentially extract maximal energy from wind. However, due to this energy extraction by such upstream or upwind turbines, the downstream or downwind turbines experience lower wind-speeds and turbulent wind conditions due to rotational motion of the rotor blades of upwind turbines. This phenomenon, widely known as ‘wake effect’, may have the following impact: (a) the amount of electrical energy realized by downwind turbines reduces considerably since power generated by wind turbines is proportional to the cube of the wind-speed, and (b) the life-time of the downstream wind turbine reduces since wake-induced turbulences increase component wear and tear at the downwind turbines.
In order to minimize the impact of the wake effects a wind farm turbine layout is designed such that there is reduced wake interaction based on statistical information regarding wind speed and wind direction in the farm. However, it may be difficult or even impossible to reduce the impact of wakes in real time when wake propagation depends on the dynamically changing wind directions and speeds.
Conventional wind farm management is mainly based on local control. That is, a wind farm controller only sends set-points regarding active and reactive power generation to individual wind turbines and then tries to vary these set-points in order to meet the demand at the point of common coupling with the electrical grid. However, wake interactions are not accounted for in the dispatch of these set-points. Moreover, local control at each individual wind turbine optimizes the active and reactive power generation only locally. Thus, while local control may be beneficial on an individual turbine level, it may not maximize energy from a farm perspective, due to the aforementioned aerodynamic wake interactions.
The local control of each wind turbine locally is mainly based on three types of mechanical actuators or wind turbine operation parameters including yaw control, pitch control, and torque control. Depending on the settings of these actuators the turbine can extract different amounts of energy, but these settings also directly influence the resulting wakes and thus the possible power generation at downwind turbines. Therefore, the overall power generation for the whole wind farm may be further improved if a coordinated behavior is ensured.
Any solution at the wind farm level has to be able to control all turbines to improve the overall power extraction, and to that purpose, account for aerodynamic wake interactions between turbines. Various studies propose different avenues to minimize wake interaction thus improving operational farm performance. For instance, controlling the amount of energy extracted by upwind turbines influences the wind energy made available for the downwind turbines. The controlling actuators or parameters include the axial induction factor, e.g. by controlling blade pitch and generator torque, yaw misalignment, or both. Pitch and torque control are commonly used in most modern variable-speed horizontal axis turbines under a maximum power point tracking perspective. Yawing the upwind turbine deliberately will deflect the wake behind the upstream turbine such that the downstream turbine is only partially located or not located in the wake of the upstream turbine anymore. As a result, the downwind turbine may potentially capture more energy thus increasing the energy capture at farm level.
The patent application EP 1790851 A2 discloses a method for operating a wind park including a central control unit receiving data from each of a plurality of wind turbines of the park. These data are used to predict load impact on downstream turbines. Control signals to selected turbines are subsequently transmitted to minimize load impact on downstream turbines and/or to reduce fatigue load of upstream turbines and to increase power capture of downstream turbines. Data received by the central control unit comprise measurements of wind velocities and directions at each turbine tower. The central control unit can use a measured change in wind conditions at an upstream turbine to send advance control demands to a downstream turbine. It is further suggested to use knowledge of wake interactions to make control decisions minimizing loads.